feat: reskin admin pages to zine design system

Migrate the entire admin section from the dark guild-* Tailwind theme
to the zine design system (dashed borders, CSS custom properties,
Brygada 1918 + Commit Mono, cream/dark mode palette).

- Replace admin top-nav layout with sidebar matching default layout
- Reskin dashboard, members, events, series management pages
- Reskin events/create and series/create form pages
- Add dev-only test login endpoint (GET /api/dev/test-login)
- Redirect duplicate admin/dashboard.vue to /admin
- Update CLAUDE.md design system docs

app/components/NaturalDateInput.vue

@@ -78,7 +78,7 @@ const props = defineProps({
     default: 'e.g., "tomorrow at 3pm", "next Friday at 9am", "in 2 hours"',
   },
   inputClass: {
-    type: String,
+    type: [String, Object],
     default: "",
   },
   required: {